Major depressive disorder affects an estimated 340 million people worldwide. According to the World Health Organization, depression is the fourth greatest public health problem. If left untreated, the effects of depression can be devastating, robbing people of the energy or motivation to perform everyday activities and, in some cases, leading to suicide. Symptoms of the disorder include feelings of sadness or emptiness, lack of interest or pleasure in nearly all activities, and feelings of worthlessness or inappropriate guilt. In addition to the personal costs of depression, the disorder also results in more than $40 billion in annual costs in the United States alone, due to premature death, lost productivity, and absenteeism.
Selective serotonin reuptake inhibitors (SSRIs) have had significant success in treating depression and related illnesses and have become among the most prescribed drugs. They nonetheless have a slow onset of action, often taking several weeks to produce their full therapeutic effect. Furthermore, they are effective in fewer than two-thirds of patients.
SSRIs work by blocking the neuronal reuptake of serotonin, which tends to increase the concentration of serotonin in the synaptic space, and thus increase the activation of postsynaptic serotonin receptors. However, although a single dose of an SSRI can inhibit the neuronal serotonin transporter and thus would be expected to increase synaptic serotonin, long-term treatment is required before clinical improvement is achieved. It has been suggested that the SSRIs increase the serotonin levels in the vicinity of the serotonergic cell bodies and that the excess serotonin activates somatodendritic autoreceptors, 5-HT1A receptors, causing a decrease in serotonin release in major forebrain areas. This negative feedback limits the increment of synaptic serotonin that can be induced by antidepressants acutely. Over time, the somatodendritic autoreceptors become desensitiized, allowing the full effect of the SSRI to be expressed in the forebrain. This time period corresponds to the latency for the onset of antidepressant activity [Perez, V., et al., The Lancet, 349:1594–1597 (1997)].
A 5-HT1A agonist or partial agonist acts directly on postsynaptic serotonin receptors to increase serotonergic neurotransmission during the latency period for the SSRI effect. Accordingly, the 5-HT1A partial agonists buspirone and gepirone [Feiger, A., Psychopharmacol. Bull., 32(4), 659–665 (1996), Wilcox, C., Psychopharmacol. Bull., 32(3), 335–342 (1996)] and the 5-HT1A agonist flesinoxan [Grof, P., International clinical Psychopharmacology, 8(3), 167–72 (1993)] have shown efficacy in clinical trials for the treatment of depression. Furthermore, such agents would also stimulate the somatodendritic autoreceptors, thus hastening their desensitization and decreasing the SSRI latency period. An agent with a dual mechanism of antidepressant action would be expected to have greater efficacy and thus reduce the number of patients refractory to treatment. Indeed, buspirone augmentation has been shown to produce marked clinical improvement in patients initally unresponsive to standard antidepressant therapy [Dimitriou, E., J. Clinical Psychopharmacol., 18(6), 465–469 (1998)].
Thus, it is highly desirable to provide improved compounds which both inhibit serotonin reuptake and which are agonists or partial agonists of the 5-HT1A receptor.